A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to impart a beam of radiation with a pattern in its cross-section, the pattern corresponding to a circuit pattern to be formed on an individual layer of the IC. This pattern can be imaged or transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an image of the entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the semiconductor manufacturing industry there is increasing demand for ever-smaller features and increased density of features. The critical dimensions (CDs) are rapidly decreasing and are becoming very close to the theoretical resolution limit of state-of-the-art exposure tools such as steppers and scanners as described above. Conventional technologies aimed at enhancing resolution and minimizing printable CD include reducing the wavelength of the exposure radiation, increasing the numerical aperture (NA) of the projection system of the lithographic apparatus, and/or including features in a patterning device pattern smaller than the resolution limit of the exposure tool so that they will not print on the substrate, but so that they will produce diffraction effects which can improve contrast and sharpen fine features.
In order to ensure that patent features applied to a substrate are applied as intended (e.g. to ensure that critical dimension limits, requirements, or uniformities are met), it may be desirable to at least partially correct for aberrations in the lithographic apparatus. Aberrations may arise due to heating of one or more elements of a projection system of the lithographic apparatus which may cause distortion or the like of those one or more elements. Previously, in order to at least partially correct for these aberrations, one or more elements of the projection system (for example, lens elements, or reflective elements) could be moved or reoriented. Movement or reorientation of these elements could be undertaken rapidly, and so the aberrations, even if time varying, could be at least partially corrected for. However, in more recent times it has been suggested to correct for such aberrations by changing the phase of one or more parts of a radiation beam passing through the projection system. The phase may be controlled using a phase adjuster, which may contain an optical element disposed in a lens pupil plane of the projection system. One or more portions of the optical element may be heated in order to change the refractive index of that portion, and thus, in use, adjust the phase of a portion of the radiation beam passing through that portion.
A problem associated with the use of phase adjusters that use heat as the means by which the phase may be controlled is that the rate of change of the applied heating (or cooling) cannot be undertaken at the same rate of change as the aberrations. This means that there is, for example, a significant error between a desired time-temperature characteristic of the portion of the optical element, and a desired time-temperature characteristic of the portion of the optical element. This error can lead to errors in pattern features applied to the substrate (i.e. pattern features not being applied as intended).